You could build some fairly insane characters in 2e as well, depending on the material you were including. (AFAIK, most of the uber builds in 3.x weren't straight from the PHB, and if you're going to include official supplements from one edition, you ought to include the them for the other.)3e is kinda tricky, because by RAW, you can have a character that mops the floor with enemies with several builds. Hardly deadly to the PCs. Then factor in you got rid of save or die, level drains, instant death at X amount of points in a single attack, and then you’re hard pressed to convince that 3e is more lethal than previous editions. In TSR D&D, a dragons breath weapon would kill most of the classes even if they made saving throws straight up
Just off the top of my head, 2e had:
-Kits (many of which had huge benefits for negligible drawbacks)
-The Complete X Handbooks (particularly Complete Humanoids, as you could play as potent monsters such as Firbolgs and Pixies)
-Skills and Powers (which allowed you to point buy your class, as well as treating most ability scores as being functionally 2 points higher than what you actually rolled)
Also, as has been mentioned, Save of Die was quite prevalent in 3e. There were lots of spells and abilities that could kill you outright, and because of the way saves scaled, a high level character might very well have a terrible chance of success (unlike 2e, where saves were a TN based on character level, meaning a high level character generally had a pretty decent chance to save vs death).
3e didn't have level drain per se, but it did have energy drain which was functionally equivalent, but in some instances easier to recover from.
I could be mistaken, but didn't 3e have a death from Massive Damage rule? Or was that optional? I admittedly don't recall anymore.
While I believe 2e also had it, 3e formalized and made standard ability score drain, which could kill or seriously incapacitate even high level characters in short order.
Dragons in 3e were intentionally designed with misleadingly low CRs so that they would be seen as tough monsters, on the assumption that PCs rarely take on a dragon blindly. 2e didn't have encounter guidelines, but if you were pitting a party against a dragon that would auto kill them, then either you didn't intend a fight or you were a sadist. 3e dragons were designed to be killers within the encounter building system, particularly for a party that wasn't given a chance to prepare.
3e did have certain stereotypical tactics that could be hard for a DM to counter, such as Scry, Teleport, Kill, but I would argue that those tactics were in no small part due to the game being very swingy and deadly at high levels. In 2e, it was unlikely that your character would be taken out by poison or death, because your saving throws were solid by that point. In 3e, it was a game of rock-paper-scissors. If the death effect targeted your good save you'd probably be fine, but if it was your poor save you'd likely want to brace for imminent character death. Hence, in 3e, it was wise to not take unnecessary risks. I believe that Scry-Teleport-Kill is still technically viable in 5e, but you never really hear people complain about it anymore (AFAIK) because high level characters in 5e are hard to kill, so they don't need to resort to such tactics in order to survive.